The present invention relates to optical imaging of turbid media, e.g. breast tissue.
Optical mammography is likely to become a relevant technology for breast cancer detection. Light scatting is the main problem that limits the potential of efficiently detecting small lesions especially when located in the central region of the breast. Structures close to surface are easily detected by optical techniques while more deep structures are more difficultly detected. A technique to preferentially probe in the central region of the breast is thus of interest.
The present text will refer by way of example only to breast tissue; however, other types of turbid media (i.e. other types of (living) tissue) may also be imaged using the techniques described herein.
Optical imaging of turbid media has been the subject of much research activity and has seen an increase in interest since the early 1990""s. This type of imaging is based on the fact that the propagation of light in a turbid medium depends on the absorption and scattering properties of the medium. Absorption results from energy level transitions of the constituent atoms and molecules in the medium. The absorption property of the medium is quantified by its absorption coefficient xcexca defined as the probability of a photon being absorbed per infinitesimal pathlength. Scattering results from variations in the index of refraction of the different structures present in the medium. In a highly diffusive medium, scattering is quantified by the reduced scattering coefficient xcexcxe2x80x2s, defined as the probability of a photon being isotropically scattered per infinitesimal pathlength. Characteristics such as intensity, coherence and polarization of the incident light change as it is absorbed and scattered by the medium resulting in diffuse transmittance of the light. In particular, scattering causes a collimated laser beam to spread over a sizeable volume element. This complicates the imaging of a turbid medium. Special imaging modalities must be implemented to offset the detrimental light diffusion. For example, time-resolved methods use short light pulses from a fast laser source to illuminate the medium. The emergent light is collected by a fast detector capable of reproducing its time variation, which can provide further information about the turbid medium. A simple data processing approach in this case is time-gating, by which only the earliest part of the output light pulses is used to produce an image. This amounts to using only the light with the straightest trajectory through the scattering medium, thus improving spatial resolution (please see J. C. Hebden and R. A. Kruger, xe2x80x9cTransillumination imaging performance: Spatial resolution simulation studiesxe2x80x9d. Med. Phys. 17, 41-47 (1990)) Reference may also be made to the following documents for additional information with respect to optical scanning techniques: S. B. Colak, D. G. Papaioannou, G. W. Hooft, M. B. Van der Mark, H. Schomberg, J. C. J. Paasschens, J. B. M. Melissen, and N. A. A. J. Van Asten, xe2x80x9cTomographic image reconstruction from optical projections in light-diffusing mediaxe2x80x9d, Appl. Opt. 36, 180-213 (1997); M. S. Patterson, B. Chance, and B. C. Wilson, xe2x80x9cTime-resolved reflectance and transmittance for the noninvasive measurement of tissue optical propertiesxe2x80x9d, Appl. Opt. 28, 2331-2336 (1989); D. Contini, F. Martelli, and G. Zaccanti, xe2x80x9cPhoton migration through a turbid slab described by a model based on diffusion approximation. I. Theoryxe2x80x9d, Appl. Opt. 36, 4587-4599 (1997); M. Morin, S. Chatigny, A. Mailloux, Y. Painchaud, and P. Beaudry. xe2x80x9cTime-domain perturbation analysis of a scattering slabxe2x80x9d, in Optical Tomography and Spectroscopy of Tissue III, B. Chance, R. R. Alfano, and B. J. Tromberg, eds., Proc. SPIE 3597, 67-78 (1999); Y. Painchaud, A. Mailloux, M. Morin, S. Verreault and P. Beaudry, xe2x80x9cTime-domain optical imaging: discrimination between scattering and absorptionxe2x80x9d, Appl. Opt. 38, 3686-3693 (1999); as well as U.S. Pat. No. 5,808,304).
The strong interest in optical imaging of scattering media stems from the need for biomedical diagnostic techniques that are safe and non-invasive. The optical properties of biological tissues are at the heart of optically based biomedical diagnostic techniques. As for the general case of a turbid medium, the manner in which light propagates through tissue depends on its absorption and scattering properties. Thus, if abnormal tissue can be said to differ from normal in its absorption or scattering of light for some physiological or morphological reason, it then becomes possible to optically differentiate between normal and abnormal conditions. A specific application is optical mammography where tumors could be differentiated from normal breast tissue on the basis of optical properties.
It would, for example, be advantageous to have a mechanism for the detection of an anomaly(ies) in turbid medium such as for example a tumour in tissue such as breast tissue.
Accordingly, the present invention in one aspect provides in a method for scanning a turbid medium for generating an image thereof for the detection of one or more anomalies contained within the turbid medium, said turbid medium having a first face and an opposite second face, wherein said turbid medium is scanned by displacing an optical signal source and a corresponding optical detector from one respective spatial location to another about the turbid medium, each spatial location being associated with a corresponding input region on said first face and a corresponding output region on the opposite dsecond face, said optical signal source directing an optical signal (e.g. laser beam) to each of said input regions, said optical detector in response to optical signals detected from each of said output regions sending corresponding detector output signal data to an image processing means for generating a set of image data of the turbid medium, the improvement comprising
generating a primary set of image data derived by scanning said turbid medium using for each input region a first input region of predetermined size and using for each output region a first output region of predetermined size, and
generating a secondary set of image data derived by scanning said turbid medium using for each input region said first region of predetermined size and using for each output region a second output region of predetermined size.
The present invention in another aspect provides in a method for scanning a turbid medium for generating an image thereof for the detection of one or more anomalies contained within the turbid medium, said turbid medium having a first face and an opposite second face, wherein said turbid medium is scanned by displacing an optical signal source and a corresponding optical detector from one respective spatial location to another about the turbid medium, each spatial location being associated with a corresponding input region on said first face and a corresponding output region on the opposite second face, said optical signal source directing an optical signal (e.g. laser beam) to each of said input regions, said optical detector in response to optical signals detected from each of said output regions sending corresponding detector output signal data to an image processing means for generating a set of image data of the turbid medium, the improvement comprising
generating a primary set of image data derived by scanning said turbid medium using for each input region a first input region of predetermined size and using for each output region a first output region of predetermined size, and
generating a further set of image data derived by scanning said turbid medium using for each input region a second input region of predetermined size greater than that of said first input region and using for each output region said first output region of predetermined size.
The present invention in another aspect provides in a method for scanning a turbid medium for generating an image thereof for the detection of one or more anomalies contained within the turbid medium, said turbid medium having a first face and an opposite second face, wherein said turbid medium is scanned by displacing an optical signal source and a corresponding optical detector from one respective spatial location to another about the turbid medium, each spatial location being associated with a corresponding input region on said first face and a corresponding output region on the opposite second face, said optical signal source directing an optical signal (e.g. laser beam) to each of said input regions, said optical detector in response to optical signals detected from each of said output regions sending corresponding detector output signal data to an image processing means for generating a set of image data of the turbid medium, the improvement comprising
generating a primary set of image data IA derived by scanning said turbid medium using for each input region a first input region of predetermined size and using for each output region a first output region of predetermined size,
generating a secondary set of image data IB derived by scanning said turbid medium using for each input region said first input region of predetermined size and using for each output region a second output region of predetermined size greater than that of said first output region and
generating a tertiary set of image data IC derived by scanning said turbid medium using for each input region a second input region of predetermined size greater than that of said first input region and using for each output region said first output region of predetermined size.
In accordance with the present invention the various sets of image data may be generated in any desired or necessary order.
In accordance with the present invention, the various sets of image data obtained as described herein may be manipulated in any manner (i.e. subjected to a data processing technique) so as to for example highlight the differences and/or similarities between the images (e.g., to obtain a composite set of image data indicative of an anomaly(ies) (e.g. breast tumour) in the body of the turbid medium); such manipulation or data processing treatment may be carried out by simple subtraction or by exploiting more sophisticated techniques such as for example K-L transforms. Thus, for example, the method(s) described herein may comprise (a) combining said primary set of image data and said secondary set of image data so as to generate a product set of image data wherein the similarities and/or differences between the said primary set of image data and said secondary set of image data are highlighted.; (b) combining said primary set of image data and said further set of image data so as to generate a product set of image data wherein the similarities and/or differences between the said primary set of image data and said further set of image data are highlighted.; (c) combining said primary set of image data said secondary set of image data and said tertiary set of image data so as to generate a product set of image data wherein the similarities and/or differences between said primary set of image data, said secondary set of image data and said tertiary set of image data are highlighted; etc. Thus, more particularly, for example, a composite set of image data may be generated by subtracting the secondary set of image data from the primary set of image data: a composite set of image data may be generated by subtracting the further set of image data from the primary set of image data; a composite set of image data may be generated in accordance with equation IB+ICxe2x88x92IA.
Preferably, the method involves combining the primary set of image data and the secondary set of image data so as to generate a product set of image data wherein similarities and/or differences between the primary set of image data and the secondary set of image data are highlighted.
In accordance with the present invention a system for performing optical imaging measurements on a turbid medium in accordance with the method(s) as described herein may, for example, comprise
any suitable (known) means for generating a collimated optical signal (e.g. laser)
directing means for directing said optical signal to an input area or region of predetermined size on a face on one side of the turbid medium (e.g. using a suitable optical fiber)
detection means for detecting a optical output signal from a second area or region of predetermined size on the face of the turbid medium on the opposite side of said turbid medium (e.g. photomuliplier tube, streak camera, etc.); the detection means may include optical fibers for directing the optical signal to the component of the detection means which may convert the optical signal to an output (e.g. analogue or digital electrical) signal; and
means (e.g. computer) for processing said optical output signals (in any known manner) so as to obtain a set of image data (e.g. data which may be projected onto the screen of a computer monitor, be printed on a printer, be further processed, stored in computer memory, etc. . . ).
The system may also of course incorporate any suitable mechanism or means for manipulating the size of the input and output regions as referred to herein.
It is to be understood herein (i.e. for the purposes of the present invention) that the optical signal(s) may be of electromagnetic radiation which is not only in the visible but as well as in the infra-red or the near infra-red part of the spectrum keeping in mind the purpose of the signal; the initial optical signal may for example be a laser beam.
The technique of the present invention may as mentioned above involve two or three (successive) scans of the turbid medium (e.g. breast tissue) so as to provide two or three projection images. These two or three images may be used to image structures according to their depth inside the breast. The first image shows all the structures as in the usual optical techniques. A second image preferentially highlights the structures near the input surface. Another, third image preferentially highlights the structures near the output surface. By a combination of two or three of the images, it is possible to highlight the structures in the central region of the breast and attenuate the structures near the surfaces.